A seemingly endless variety of plastic goods is manufactured through the use of known rotational molding equipment and processes. Very generally, such goods frequently are produced on a turret-like apparatus having a multiplicity of axially rotatable arms each of which extends radially outward from the central body of the apparatus and receives at the outermost arm end a removable cast and/or machined hollow matrix for shaping a plastic compound into a useful object. The arms of the turret-like apparatus and the molds attached thereto also rotate in a horizontal plane about the central vertical axis of the turret apparatus and in a sequential manner through a variety of process stations which are located circumferentially thereabout. The stations typically encountered include a loading station, an oven station, a holding station, a cooling station and an unloading station. At the loading station, a mold equipped with a vent tube and containing an elastomeric substance is attached to one of the radially extending arms of the turret-like apparatus. At the oven station, the mold is received and maintained for a predetermined time within a chamber where heat energy is transferred to the mold while the mold is simultaneously caused to rotate as a result of axial rotary movement imparted to the turret arm by the turret apparatus. During the time that the mold is within the oven chamber, air within the cavity of the mold heats and expands, creating increased pressure which traditionally has been kept below a desired maximum level by allowing a portion of the heated air to escape through the mold vent tube. Once the curing operation has been completed, the mold is removed from the oven station and rotatably advanced either to the holding station or to the cooling station. After the mold and the object produced therein have undergone sufficient cooling, the mold is further advanced to the unloading station where the mold is opened and the cured object is removed therefrom. The molding process may then be repeated to produce another object after a new supply of elastomeric material is introduced into the mold.
The vent tubes which have been used with the above described molding equipment and processes have generally been very simple and inexpensive structures. In virtually all instances, they have been comprised of a generally straight piece of pipe-like conduit being open at each of its ends and further being axially unobstructed over its entire interior length. The vent tubes traditionally been made from a heat resistant material which can withstand the heat energy transferred to them not only from the hot gas that escapes the mold cavity, but also from the mold itself and from the heat transfer medium contained within the oven chamber. While a number of heat resistant materials have proven suitable over the years, the preferred material of late has been polytetrafluoroethylene (PTFE).
Aside from being simple, straight pieces of PTFE conduit, known vent tubes also have been created by machining a length of PTFE conduit so that it will have various external and internal wall surface features. One such tube has a constant diameter internal bore and a dual-diameter outer wall surface. The larger diameter outer wall surface extends over the length of the tube that makes contact with that portion of the mold body defining the orifice into which the vent tube is inserted and that is exposed to the heat transfer medium of the oven chamber. The larger diameter outer wall surface also includes at its midsection a flange that extends radially outward and serves as a ring-like stop to limit movement of the vent tube into the mold body. Another such tube has an external wall surface that consists of three distinct regions. The first region is one that begins at the outermost end of the tube, i.e., the tube end that is exposed to the oven chamber, and terminates at a point along the tube periphery that adjoins the entrance to the vent tube orifice of the mold. The first region has a diameter that is constant throughout and that is larger than the diameter of the vent tube orifice of the mold. The second region immediately adjoins the first region and extends inwardly toward the interior cavity of the mold for a distance that is substantially equivalent to the thickness of the mold wall at the vent tube orifice. Since the second region is smaller in diameter than the first region, a step-like flange is formed at the junction of the two regions. In a manner similar to that of the ring-like stop provided on the other of the above described machined vent tubes, the step-like flange also functions to limit movement of the vent tube into the mold body. The third region extends directly from the second region and into the mold cavity. The diameter of the third region gradually decreases from that of the second region where the second and third regions meet one another to a slightly lesser diameter at the innermost end of the vent tube. This gradual diameter decrease provides the third region with a gently tapered exterior surface when it is viewed from the side. Like the other machined vent tube described hereinabove, the second machined tube has a internal bore with a uniform internal diameter. The interior bore may be provided with a circumferentially threaded region that extends from the outermost end of the tube inwardly within the first region for a distance substantially equal to the first region's length. The circumferentially threaded region serves as a means to couple an air supply line to the vent tube and to thereby provide an external air source for pressurizing the interior of the mold during the curing cycle. Typically, pressurization is 1 psi or less.
Traditionally, mold vent tubes of the type just described are provided with a quantity of woven or matted material such as steel wool or fiber glass which is inserted into the interior bore of the vent. Such material serves a dual purpose. First, it provides a barrier to help prevent loss of the elastomeric substance from the mold cavity during the molding process. And second, it permits outward flow of hot gas from the mold cavity.
The known vent tubes so equipped with steel wool or fiber glass have functioned satisfactorily for their intended purpose; however, one of the drawbacks associated with the use of such tubes is the need to service them at the end of each curing cycle. Typically servicing entails manually removing, cleaning, repacking and reinserting them into the mold. These tasks are made necessary, because during the curing cycle a small amount of the elastomeric substance flows from the mold cavity into the vent tube and becomes entrapped in the steel wool or fiber glass. Even though the amount of elastomeric substance that becomes entrapped is small, it causes enough clogging of the steel wool or fiber glass to prevent further passage of hot gas during the next cure cycle in a volume that will avoid the occurrence of molding defects in the object being formed by the mold. Servicing of the conventional vent tubes has typically been done by a worker who loads and unloads each of the molds before and after each curing operation. As may be expected, such servicing adds to production time, thereby reducing productivity. It also increases overall production cost due to the additional labor required.
In view of the foregoing, it should be evident that a need exists for a vent tube assembly which does not need to be serviced after each curing cycle. Therefore, it is an object of the present invention to provide a new and useful rotational mold vent tube assembly which may be used for a substantial number of curing cycles before the assembly must be removed from the mold and serviced.
It is also an object of the present invention to provide a new and useful rotational mold vent tube assembly that will increase manufacturing productivity by shortening the time required to ready a mold for use and begin a new curing cycle.
It is a further object of the present invention to provide a new and useful rotational mold vent tube assembly that will reduce overall labor costs.
It is yet a further object of the present invention to provide a new and useful rotational mold vent tube assembly comprised of a material that will make the assembly more heat resistant and durable.